Recently, as demand for portable information devices such as smart phones drastically increases, required is developing a nano-sized nonvolatile memory (NVM) device capable of implementing ultra large scale integration (terabit), an ultra-miniaturized size, a high speed and high reliability.
However, as the nonvolatile memory device becomes small and integrated, a data retention capability is lowered due to leakage current, and power consumption efficiency is low due to a high operation voltage.
Accordingly, has been proposed a polySilicon-Oxide-Nitride-Oxide-Silicon (SONOS) type memory device having a memory characteristic of a high performance and compatible with the conventional processes, as the next generation nonvolatile memory device. However, such SONOS memory device also has the following disadvantages.
Firstly, in a case where a tunneling insulation layer formed of a silicon oxide film of a single layer has a reduced thickness for an enhanced operation speed, it cannot obtain a data retention characteristic more than 10 years, a characteristic of a nonvolatile memory, due to increase of direct tunneling and stress induced leakage current. If the thickness of the silicon oxide film is increased, a data retention characteristic more than 10 years can be obtained. However, in this case, an operation speed may be degraded, and a high voltage may be required.
Secondly, when performing an erasing operation using a blocking insulation layer formed of a silicon oxide film, electrons may be injected into a nitride film from a gate due to increase of an erasing voltage. This may cause the erasing operation not to be performed perfectly.
In the conventional non-volatile memory device having a SONOS structure, if a program voltage is increased for enhancement of a program speed, power consumption may increase, and defects inside the tunneling oxide film may increase. Further, if the thickness of the tunneling oxide film is reduced for an enhanced program speed while the program voltage is constantly maintained, leakage current may occur. This may cause reliability of the non-volatile memory device not to be obtained. Therefore, it was impossible to enhance the program speed with low power, while obtaining reliability on data retention.